Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Thu Sep 10 2020 Valid 12Z Sun Sep 13 2020 - 12Z Thu Sep 17 2020 ...Overview and Guidance/Predictability Assessment... Most of today's guidance offers similar ideas for the large scale forecast through the period. A complex evolution over the eastern Pacific should yield a fairly deep trough (with an upper low becoming increasingly likely) that approaches the West Coast. Meanwhile initial western U.S. and southeastern U.S./western Atlantic ridges should combine to produce a broad area of southern tier mean ridging, though with one or more embedded weaknesses. Currently the best potential for such features will be over or near the Plains and in the vicinity of the northeastern Gulf of Mexico. Progressive flow will prevail across southern Canada and most of the northern U.S. The primary focus of guidance trends and spread is on the eastern Pacific trough along with some downstream effects across the northern U.S./southern Canada. Over the past two or three days there has been pronounced trending in the guidance, first toward a deeper trough and then for the closing of an upper low. Involvement of multiple pieces of energy (an initial East Pacific upper low and one or two digging shortwave impulses from the northeastern Pacific/Gulf of Alaska) constrains confidence in specifics. Broadening ensemble spread by days 6-7 Wed-Thu also highlights the uncertainty. Perhaps due in part to the 18Z/00Z GFS runs bringing more energy through a Gulf of Alaska ridge around Tue, those runs bring the upper trough farther into the western U.S. than most other guidance including recent GEFS means. In addition those GFS runs are faster than consensus to eject leading energy (a portion of the East Pacific upper low) into the westerlies, leading to a fast progression of the wave that tracks along/north of the U.S.-Canadian border east of the Rockies. The new 00Z ECMWF/CMC seem to further the recent trends, keeping the Pacific upper low farther offshore. However the 00Z ECMWF amplifies southern Canada/northern U.S. troughing more than other guidance by next Wed-Thu. With the 12Z GFS/GEFS mean generally comparing better to other guidance than later runs from the East Pacific through the western/northern U.S., the updated forecast started with a 12Z operational model blend for the first half of the period and then introduced some 12Z GEFS/ECMWF mean input to complement the GFS/ECMWF/CMC as detail uncertainties increased. This blend provided the best representation of latest trends (before the 00Z cycle) along the West Coast. It kept good continuity over the eastern/southern states as guidance has been very consistent over the past couple days or so with the frontal system crossing the eastern states and then settling near the Gulf Coast as trailing high pressure builds over the East. ...Weather/Hazard Highlights... The front crossing the eastern U.S. on Sun will bring a broad area of rainfall with some locally moderate-heavy amounts, followed by a period of drier weather over the northern half to two-thirds of the East as strong high pressure builds in. On the other hand the combination of initial southern tier moisture and the aforementioned front settling along the Gulf Coast for multiple days should ultimately promote periods of rainfall through the period between the Southeast and Texas with some significant totals possible. A weak upper feature/surface trough over portions of the northern-northeastern Gulf may further enhance rainfall. The overall precipitation area could expand northward over the Tennessee/Mississippi Valley regions by the latter half of the period depending on how the potential upper weakness over or near the Plains evolves. The upper trough/low evolving off the West Coast may bring meaningful rainfall to some areas along the northern half of the West Coast. Ongoing spread/trends in the guidance continue to temper confidence in timing, coverage, and amounts though. As the initial western U.S. upper ridge moves into the Plains Sun into the next work week, expect warmest temperatures versus normal (up to plus 10-15F anomalies) to progress from the Interior Northwest into the Northern Plains followed by a slight decrease in greatest anomalies from the north-central Plains into the Rockies/Great Basin toward Wed-Thu. The approaching East Pacific upper trough will bring a cooling trend but to what extent remains in question. Morning lows should remain above normal over much of the West. Highs over the East will tend to be near to below normal due to the clouds/rain over the southern tier during the period and the front crossing the East early, as well as with cool high pressure building in behind the front. Flow ahead of the front will bring above normal lows over much of the East Sun-Mon. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml